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| United States Patent |
5,236,744
|
|
Suga
, et al.
|
August 17, 1993
|
Coating method
Abstract
A slide-hopper-type coating method in which the viscosity of the coating
solution forming a free-falling curtain and the angle .alpha. which the
front edge of the lip of the slide hopper forms with respect to the
horizontal are selected with respect to the flow rate of the curtain in
such a way that the line of contact between the curtain and the web is
concave with respect to the direction in which the web travels. In
addition, the angle .beta. which the extension of the curtain in the
direction of its fall forms with respect to the direction of travel of the
web at the point where the curtain is deposited on the web is preferably
adjusted so that it is an obtuse angle not larger than 140.degree.. By
satisfying these conditions, the coating method of the present invention
is capable of extending the upper limit of coating speeds without causing
"sagging" in the range of high flow quantities of coating solution per
unit length of curtain coat width.
| Inventors:
|
Suga; Yasushi (Kanagawa, JP);
Kobayashi; Kiyoshi (Kanagawa, JP);
Sasahara; Toshimitsu (Kanagawa, JP);
Miyamoto; Kimiaki (Kanagawa, JP)
|
| Assignee:
|
Fuji Photo Film Co., Ltd. (Kanagawa, JP)
|
| Appl. No.:
|
605297 |
| Filed:
|
October 30, 1990 |
Foreign Application Priority Data
| Current U.S. Class: |
427/420; 118/DIG.4 |
| Intern'l Class: |
B05D 001/30 |
| Field of Search: |
427/420
118/DIG. 4
|
References Cited
U.S. Patent Documents
| 3508947 | Apr., 1970 | Hughes | 118/324.
|
| 3632374 | Jun., 1968 | Greiller | 118/324.
|
| 4135477 | Jan., 1979 | Ridley | 118/325.
|
| 4842900 | Jun., 1989 | Miyamoto | 427/348.
|
| Foreign Patent Documents |
| 2285931 | Sep., 1975 | FR.
| |
| 50-76151 | Jun., 1975 | JP.
| |
| 62-197176 | Aug., 1987 | JP.
| |
| 64-51170 | Feb., 1989 | JP.
| |
| 1-051170 | Feb., 1989 | JP.
| |
| 1-131549 | May., 1989 | JP.
| |
Primary Examiner: Padgett; Marianne
Attorney, Agent or Firm: Sughrue, Mion, Zinn, Macpeak & Seas
Claims
What is claimed is:
1. In a coating method in which a curtain of coating solution free falling
from a slide hopper coating head impinges against a continuously running
web, the improvement wherein a viscosity of said coating solution being
applied and an angle .alpha., which a front edge tip of a lip of said
slide hopper forms with respect to the horizontal, are selected with
respect to a flow rate of a free-falling curtain of said solution in such
a way that a line of contact between said curtain and said web is concave
with respect to a horizontal direction in which said web travels, said
line having a center point which is at least 3 mm further in said
horizontal direction of said travel
2. The coating method according to claim 1 wherein an angle .beta., which a
line extending along a falling direction of said free-falling curtain
forms with respect to said web at a said center point, is an obtuse angle
not larger than 140.degree..
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method by which various liquid
compositions are curtain-coated onto a continuously running support in
strip form (hereinafter referred to as a "web") in the manufacture of
photographic films, photographic papers, magnetic recording tapes,
adhesive tapes, pressuresensitive recording papers, offset printing
plates, or the like.
2. Background of the Related Art
The basic technology of curtain coating is described in U.S Pat. Nos.
3,508,947 and 3,632,374. In "AIChE Winter National Meeting" (1982), S. F.
Kistler disclosed the theory of curtain coating, focusing on the following
three phenomena which he considered would govern the rate of application
by curtain coating:
(1) incorporation of tiny air bubbles between the web and the coating
solution (which phenomenon is hereinafter referred to as "air
entrainment");
(2) formation of a liquid deposit along the line where the coating solution
contacts the web (which phenomenon is hereinafter referred to as "heel"
and is common when the coating solution flow rate is large); and
(3) the coating solution is not adequately deposited but will bounce back
from the web being coated (which phenomenon is caused by "heel with air
entrainment" and is hereinafter referred to as "sagging" and i also common
when the coating solution flow rate is large).
According to Kistler, curtain coating is no longer possible if one or more
of these phenomena occur.
Various attempts have previously been made to increase the curtain coating
speed limited by the aforementioned phenomena. They include:
(1) replacing the web-entrained air layer with carbon dioxide to suppress
the phenomenon of "air entrainment" (see U.S. Pat. No. 4,842,900); and
(2) applying a static electric field between the web and the coating
solution, whereby the adhesion of the latter is enhanced to suppress the
phenomenon of "air entrainment" (see Unexamined Published Japanese Patent
Application No. 197176/1987).
In fact, however, as modern coating plants adopt application speeds of 250
m/min and higher with the curtain of coating solution flowing in higher
rates, the limitation of coating speeds by "heel" and "sagging" has become
a greater concern than the limitation by "air entrainment". A method that
has previously been proposed for dealing with this problem is:
(3) suppressing the phenomenon of "heel" by properly adjusting the shearing
viscosity between the lower and upper layers of coating solution (see
Unexamined Published Japanese Patent Application No. 131549/1989).
A method of curtain coating has also been proposed for insuring that a
curtain forms consistently, even if the coating solution is allowed to
flow in comparatively low rates, to thereby prevent the thickness of the
curtain from being unduly increased at the two lateral edges, which method
is:
A multiple curtain coating including the steps of forming at least two
layers of a coating solution on an inclined sliding surface, allowing the
superposed layers to fall down a curved lip portion at the terminal end of
the sliding surface, and permitting the resulting free-falling curtain to
be deposited on a running support, wherein the angle .beta. at which the
sliding surface is inclined with respect to the horizontal is
30.degree.-70.degree.. This surface at which the curved lip portion
contacts the superposed layers of coating solution is a cylindrical
surface having a radius of curvature of at least 40 mm. This method may be
modified such that the direction in which the curtain falls down forms an
angle .beta. of 120.degree.-150.degree. with respect to the direction of
travel of the web at the point where the curtain is deposited on the
support (see Unexamined Published Japanese Patent Application No.
51170/1989). The techniques described in Unexamined Published Japanese
Patent Application No. 51170/1989 and 131549/1989 are such that the flow
rate of the curtain of coating solution is in the range of 1.0-4.0
cm.sup.3 /cm.multidot.sec (the unit length of coating width being
expressed in centimeters). Those techniques are effective in the specified
range for forming a consistent curtain and preventing its thickness from
being unduly increased at both lateral edges. However, no study has been
made to determine whether they are effective in suppressing the phenomenon
of "sagging" in flow rates exceeding 4 cm.sup.3 /cm.multidot.sec.
SUMMARY OF THE INVENTION
An object, therefore, of the present invention is to solve the
aforementioned problems of the prior art by providing a coating method
that is capable of extending the upper limit of coating speeds without
causing "sagging" at high flow rates in the range of 3-7 cm.sup.3
/cm.multidot.sec (the unit length of curtain coating width being expressed
in centimeters).
In the course of intensive studies conducted to attain the aforementioned
object, the present inventors noted that the shape of the line Of contact
between the free-falling curtain of coating solution and the web was
important. The present invention was accomplished on the basis of this
finding.
The aforementioned object of the present invention can be attained in a
coating method which consist of allowing a free-falling curtain of coating
solution to impinge against a continuously running web, wherein the
viscosity of the coating solution and the angle .alpha., which the front
edge of the lip of a slide hopper forms with the horizontal are selected
with respect to the flow rate of the free falling curtain in such a way
that the line of contact between the free falling curtain and the web is
concave with respect to the direction in which the web travels, the degree
of concavity being at least 3 mm. Preferably, the angle .beta., which an
extension of the free falling curtain in the direction of its fall forms
with respect to the direction of travel of the web at the point where the
curtain is deposited on the web, is an obtuse angle not larger than
140.degree..
The novel characteristic feature of the present invention is that the
viscosity of the coating solution being applied and the angle which the
front edge of the lip of a slide hopper forms with respect to the
horizontal are preselected with respect to the flow rate of the free
falling curtain of the coating solution, in such a way so that the line of
contact between the free falling curtain and the web is concave with
respect to the direction in which the web travels, with the degree of
concavity being at least 3 mm.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1(a) and (b) are is a perspective view illustrating two shapes of the
line of contact between a web and a free-falling curtain that is formed of
the coating solution supplied from a slide hopper;
FIG. 2 is a side view illustrating various dimensions of a slide hopper
type curtain coater a they relate to the present invention; and
FIG. 3(a) and (b) are partial side views illustrating the angle .beta.
which the extension of the free-falling curtain in the direction of its
fall forms with respect to the direction of travel of the web at the point
where the curtain is deposited on the web.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will now be described below in further detail with
reference to the accompanying drawings. FIG. 1(a) is a perspective view
showing a concave line of contact between the web and the free-falling
curtain of coating solution that is flowing down a slide hopper, and FIG.
1(b) is a perspective view showing a convex line of contact between the
web and the free-falling curtain.
As shown, the coating solution flowing down the inclined surface of a slide
hopper 1 is supported by rods 4 along two sides to form a free-falling
curtain 5, which impinges against the web 3 to be coated thereon along the
line of contact 6.
According to the present invention, the line of contact is made concave, as
indicated at 6a in FIG. 1(a), and the degree of concavity x, as measured
at the center of the line of contact, is no smaller than 3 mm. By forming
a free-falling curtain that satisfies these requirements, consistent and
rapid curtain coating can be accomplished. The condition of x.gtoreq.3 mm
means that the distance from the line connecting the two supporting rods
at an end to the bottom of the concave portion at the center of the
curtain should be at least 3 mm. If x<3 mm, the purpose of the present
invention (i.e., increasing the upper limit of coating speed) cannot be
fully achieved.
FIG. 2 is a side view showing how curtain coating is performed with the
coating solution supplied from a slide hopper. In the present invention,
the angle .beta., which is formed between the direction in which the
free-falling curtain 5 flows down and the direction of travel of the web 3
at the point where the curtain is deposited on the web, may be an obtuse
angle not larger than 140.degree., which is effective for meeting the two
requirements described above, namely, the line of contact between the
free-falling curtain 5 and the web 3 should be made concave with respect
to the direction of travel of the web, and the degree of concavity should
be at least 3 mm.
In the present invention, the angle .alpha. which the front edge 7 of the
lip of the slide hopper 1 forms with respect to the horizontal can be
adjusted by modifying the shape of the front edge 7. Through adjustment of
the angle .alpha., not only the flow rate at which the coating solution
leaves the slide hopper 1 to form the free-falling curtain 5 but also the
shape of the curtain will change. The angle .alpha. can also be adjusted
by changing the inclination of the slide hopper. This technique of
changing the inclination of a fluid supplier has not been practiced with
conventional curtain coaters.
In the present invention, the angle .beta. at which the free-falling
curtain is deposited on the web may be adjusted, by changing the position
of roller 2 or slide hopper 2, which allows the curtain to be deposited on
the web at different points, as indicted at 6 in FIGS. 3(a) and 3(b). In
FIG. 3(a), .beta. is nearly equal to 90.degree., and in FIG. 3(b), .beta.
is nearly equal to 140.degree.. If .beta.>140.degree., the free-falling
curtain is unable to form a consistent line of contact 6 with the web.
Selection of the proper point of deposition requires the fluid supplier to
be precisely adjusted in the direction of travel of the web.
Through precise adjustment of the viscosity of the coating solution and the
inclination of the slide hopper as well as its distance to the web in
either the horizontal or vertical direction, the upper limit of coating
speeds can be extended.
EXAMPLE
An aqueous solution of 10% alkali-processed gelatin containing 0.15 wt% of
an anionic surfactant (sodium salt of 2-ethylhexyl .alpha.-sulfosuccinate)
was treated with poly(sodium styrenesulfonate) so as to increase its
viscosity to 30 cps or 60 cps at a shear rate of 10 sec.sup.-1. The
thus-prepared samples of coating solution were applied onto webs
(polyethylene-laminated papers) using a slide hopper, with the flow rate q
of the free-falling curtain per unit length of the coat width being varied
at 2, 3, 4 or 6 (cm.sup.3 /cm.multidot.sec). During the application, the
distance h between the lip portion of the slide hopper and the point at
which the curtain was deposited was held at 100 mm; on the other hand, the
angle .alpha. which the front edge of the lip of the slide hopper formed
with the horizontal was varied at about 435.degree., 90.degree. and
120.degree., whereas the angle .beta. at which the curtain was deposited
on the webs was varied at about 90.degree. and 120.degree.. the results of
comparison of the coating speeds that could be achieved in these various
cases are shown in Table 1.
TABLE 1
______________________________________
Run Flow Limit cast-
No. quantity Viscosity
x .alpha.
.beta.
ing speed
______________________________________
1 3 30 11.0 45 90 210
2 6 30 28.0 120 90 340
3 4 60 7.5 45 90 320
4 4 60 9.5 90 90 350
5 4 60 11.6 90 125 400
6 2 60 0 45 90 300
7 6 30 -10 45 90 150
8 6 60 -7 45 90 200
______________________________________
Units of measurement: flow rate, cm.sup.3 /cm.multidot.sec; viscosity, cps;
x, mm; coating speed, m/min; .alpha., .beta., degrees.
Symbol x represents the distance of the bottom or peak at the center of the
line of contact as measured from the line connecting the support rods on
two sides of the web; when x>0, the line of contact was concave with
respect to the direction of travel of the web, and when x<0, the line of
contact was convex.
Table 1 shows the following:
(1) Given the same flow rate, the limit coating speed increased with
increasing concavity of the line of contact when the viscosity of the
coating solution was higher than a certain value (compare Runs Nos. 3, 4
and 5), which allowed the coating operation to be performed at higher
speeds.
(2) When the viscosity of the coating solution was low for a given flow
rate, the line of contact was convex, thereby reducing the limit coating
speed.
(3) The more the angle .alpha. increased beyond 45.degree., the more likely
it was for the line of contact to become concave (compare Runs Nos. 3 and
4); and
(4) The line of contact was more likely to become concave when .beta. was
125.degree. than when it was 90.degree..
In the present invention, the viscosity of the coating solution forming a
free-falling Curtain and the angle .alpha. which the front edge of the lip
of a slide hopper forms with respect to the horizontal are selected with
respect to the flow rate of the curtain in such a way that the line of
contact between the curtain and the web is concave with respect to the
direction in which the web travels. In addition, the angle .beta. which
the extension of the curtain in the direction of its fall forms with
respect to the direction of travel of the web at the point where the
curtain is deposited on the web may be so adjusted that it is an obtuse
angle not larger than 140.degree.. By satisfying these conditions, the
coating method of the present invention is capable of extending the upper
limit of coating speeds without causing "sagging" in the range of high
flow rates of coating solution per unit length of curtain coat width,
which contributes to a marked improvement in productivity.
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